The cyst wall of Entamoeba invadens contains chitosan (deacetylated chitin)

The cyst wall of Entamoeba invadens (Ei), a model for the human pathogen Entamoeba histolytica, contains chitin, which is a homopolymer of beta-1, 4-linked N-acetyl-glucosamine (GlcNAc). In fungi and in bacteria that make nodulation factors, chitin deacetylases make chitosan, which is a mixture of GlcNAc and glucosamine and so has a positive charge. The activity of an Ei chitin deacetylase was revealed by a 3-4-fold increase in released GlcNAc when deproteinated cyst walls were chemically acetylated prior to treatment with a commerical chitinase. Because this chitinase releases GlcNAc but not GlcN, increases in released GlcNAc after acetylation suggested the presence of chitosan in Ei cyst walls. Five putative Ei and Eh chitin deacetylase genes resembled those of fungi and bacteria. A predicted Eh chitin deacetylase matched closely the three-dimensional structure of a Bacillus subtilis peptiodglycan deacetylase. A recombinant Eh chitin deacetylase, expressed in Saccharomyces cerevisiae, deacetylated chitooligosaccharides in vitro. These results are consistent with the idea that Ei chitin deacetylases modify chitin to produce chitosan in the Ei cyst wall.

Gene discovery in the Entamoeba invadens genome

Entamoeba invadens, a parasite of reptiles, is a model for the study of encystation by the human enteric pathogen Entamoeba histolytica, because E. invadens form cysts in axenic culture. With approximately 0.5-fold sequence coverage of the genome, we were able to get insights into E. invadens gene and genome features. Overall, the E. invadens genome displays many of the features that are emerging from ongoing genome sequencing efforts in E. histolytica. At the nucleotide level the E. invadens genome has on average 60% sequence identity with that of E. histolytica. The presence of introns in E. invadens was predicted with similar consensus (GTTTGT em leader A/TAG) sequences to those identified in E. histolytica and Entamoeba dispar. Sequences highly repeated in the genome of E. histolytica (rRNAs, tRNAs, CXXC-rich proteins, and Leu-rich repeat proteins) were found to be highly repeated in the E. invadens genome. Numerous proteins homologous to those implicated in amoebic virulence, (Gal/GalNAc lectins, amoebapores, and cysteine proteinases) and drug resistance (p-glycoproteins) were identified. Homologs of proteins involved in cell cycle, vesicular trafficking and signal transduction were identified, which may be involved in en/excystation and cell growth of E. invadens. Finally, multiple copies of a number of E. invadens genes coding for predicted enzymes involved in core metabolism and the targets of anti-amoebic drugs were identified.

LINEs and SINE-like elements of the protist Entamoeba histolytica

A survey of whole genome shotgun sequences of the protozoan parasite Entamoeba histolytica revealed three families of non-long terminal repeat (LTR) retrotransposons or long interspersed elements (LINEs) (called EhLINEs in this report). The 4.8 kb EhLINEs each had a single open reading frame with a putative nucleic acid binding motif (CCHC) and restriction enzyme-like endonuclease domain located downstream of the reverse transcriptase (RT) domain. Phylogenetic analysis of the RT domain placed the EhLINEs in the R4 clade of non-LTR elements, a mixed clade of non-LTR elements that includes members from nematodes, insects, and vertebrates. EhLINE1 (which was previously identified as HMc and EhRLE) shared a common 3' end with a highly transcribed 0.55 kb short interspersed element (SINE)-like element previously identified as IE or ehapt2 and called EhLSINE1 in this report. Similarly, EhLINE2 shared a common 3' end with a highly transcribed 0.65 kb SINE-like element called EhLSINE2 in this report. The shared 3' end sequences of the EhLINEs and EhLSINEs suggested that EhLINEs are involved in the retrotransposition of the EhLSINEs. EhLSINEs were flanked by target site duplications and contained conserved 5' sequences, which likely regulate their transcription. The EhLSINEs are the first protist SINE-like elements identified that share a common 3' sequence with LINEs, and the first SINE-like elements that have been associated with the R4 clade of non-LTR elements.

Entamoeba histolytica lectins contain unique 6-Cys or 8-Cys chitin-binding domains

The Jacob lectin, the most abundant glycoprotein in the cyst wall of Entamoeba invadens, contains five unique 6-Cys chitin-binding domains (CBDs). We identified Entamoeba histolytica and Entamoeba dispar genes encoding Jacob homologues, each of which contains two predicted 6-Cys CBDs. A unique 8-Cys CBD was found at the N termini of the E. histolytica chitinase and three other predicted lectins, called Jessie 1 to Jessie 3. The CBDs of four E. histolytica lectins (Jacob, chitinase, and Jessies 2 and 3) were expressed in secretory vesicles of transfected amebae and shown to bind to particulate chitin.